The standard explanation (for the lack of evidence) which I've seen in history books prior to ever hearing about Illig, goes like this more or less: that starting somewhere around 900 AD or thereabouts, you had a period of at least a hundred years during which Moors, Vikings, and Magyars all three just rode roughshod over all of Europe, particularly Magyars who are said to have made two or three round trips through Europe raping, pillaging, plundering, and stealing just everything, leaving Europe a tabula Raza after which and after things got back under control somewhat, you had the age when the present cathedrals and what not were built.
The idea of a dark age, which we DO read about, basically means a historical age for which physical evidence does not exist or exists in such minute and questionable quantities as to provide no useful information. The simplest possible explanation for such an age, which people like Illig and I assume Gunnar Heinsohn subscribe to, is that it simply never happened.
The standard explanation of "dark age" is ... that there was little to no evidence in (written) documents.
Thus, this period differs from region to region - actually: differed, since nowadays we've got a lot of sources from all periods.
At least in this (larger) part of Germany neither before, nor around nor after 900 we had no Moors, Vikings (leaving aside the various attacks of the Vikings on a couple of places along the Rhine up to Cologne and Prüm in the Eifel), and Magyars here.
Actually, the Magyars around 900 were here:
So you think, all the 'stuff' I've seen, touched, worked with, read ... all that wasn't from that time? My native town and all the others around here didn't exist? The Saxon wars against the Franks didn't happen? ...